Pumps are the most frequent machines in engineering. In technical literature hydraulic pumps as well as ventilators and blast engines working with low pressure ratios are classed as "pumps". During the designing of the machine there is this no need to take the compressability of the fluid to be delivered into consideration.
Both the positive displacement pump as well as the fluid flow pump follow the known model laws, i.e. for the positive displacement pump it holds that P.about.n.multidot.D.sup.3 and for the fluid flow pump it holds that P.about.n.sup.3 .multidot.D.sup.5, where P is power, n is speed of rotation and D are the characteristic dimensions of the energy-transfering module of the machine. It is evident that the power of a positive displacement pump increases linear with the speed of rotation, whereas the power of a fluid flow pump increases with the third power of the speed of rotation. In the following we refer to fluid flow pumps, although the invention also relates to the two types of machines mentioned above. The model laws illustrate the effect of speed on the hydraulic power of the machine in question. Consequently, it is of considerable advantage with regard to dimensions, weight, price and often efficiency of a pump assembly to operate a pump at high speeds.
When driving a pump by means of an electromotor the speed of the pump is in most cases directly dependent on the frequency of the main circuit. That is the reason why frequency converters are increasingly employed. Such a converter has further advantages. It allows, for example, the simultaneous operation of structurally equal assemblies of different speeds for the execution of different tasks while at the same time reducing the stock of spare parts. Furthermore the user is no longer forced to exactly compute the characteristic curve of the assembly in advance, since the requirements of the assembly are met substantially lossless by choosing the correct speed. Finally, it is possible to deliver various products in the same system without exchanging the assembly by simply altering the speed. This is often necessary in chemical plants.
A prerequisite for providing these advantageous is the installation of frequency converters. Known converters have to be installed apart from the pump assembly, since they are bulky and expensive. The price of a frequency converter normally increases the price of a pump assembly considerably, especially in the low-power range. A further disadvantage of known converters is interference with the ambient due to electromagnetic fields generated by the cable between a frequency converter and a pump assembly. This is only avoidable by extensive shielding, thus further curtailing the mobility of the pump assembly.